Wireless communication technologies continue to evolve to meet the demand for increased data throughput. This is addressed on many levels with different approaches including higher order modulation, multiple-input and multiple-output (MIMO), scheduling, increased bandwidth, and so on. One way of meeting the challenging capacity gains in next generation wireless networks (e.g. 5G) is by supporting large communication bandwidth. The frequency utilization is likely to span several bands, often denoted communication on non-contiguous intra-band as well as inter-band. Such trend in the telecommunication systems can be solved by multiple parallel receiving paths and is the most common implementation in the literature to date.
One common solution to combine multiple radio frequency bands in a wideband signal is to use a multiband receiver supporting simultaneously two or more frequency bands. The most straightforward way of doing that is to implement the multiband receiver with an entire radio frequency receiver for each radio frequency band. Each of these parallel receivers is then designed to receive a specific radio frequency band. This is the most common solution of the multiband support, where few modifications are needed on top of a copy paste action on an available single frequency band receiver.
Although this solution can be implemented by just using a number of well-known single frequency band receivers in parallel, it also has several disadvantages.
First, it is area consuming as baseband (or low-IF) blocks are replicated. For example, filtering capacitors become large when operating on low bandwidth signals. For the same reason, it is also quite power consuming since multiple base band stages are needed.
Of further disadvantages, it can be mentioned that there is a need for unique and separate local oscillator signal generation for each frequency translating mixer. The local oscillator generation will consume considerable power, and more importantly, unavoidable interaction between these different local oscillator frequencies can cause system performance degradation.
Each mixer should be a quadrature mixer, which will generate imbalance between I and Q path. Any such I/Q imbalance may e.g. cause the image of a first signal in a first band to at least in part overlap a second (desired) signal in the first band or in a second band and thus inhibit or reduce the ability to detect second signal. I/Q-imbalance requirement may call for calibration, which is then needed for each receiver branch. Such estimation and compensation procedure can only be achieved with the wanted accuracy in the digital base band. Each mixer should have its own I/Q imbalance calibration, which is very hard to implement, especially if it becomes frequency dependent and the number of parallel branches increases.
An alternative solution that introduces the idea of reusing some of the receiver blocks is the combination of the narrower signals after the down converting stage, i.e. at base band or low intermediate frequency band. This implementation allows reducing the impact of the multiple routing since a virtual ground is available already at base band, if a direct conversion receiver using a well-established current passive mixer topology is chosen. Since the baseband (or low-IF) blocks are no longer replicated, this solution is less area consuming, and for the same reason, it is also less power consuming. However, the other disadvantages mentioned above are still present also in this solution.
In some solutions, the signals may be added at radio frequency in power, using power combiners. However, also combining the signals in power at radio frequency has some drawbacks. Summation in power will introduce loss in the combiner when supporting isolation between the ports and supporting a rich span of frequencies, i.e. frequency selective power combiners are not considered applicable for this scenario. Further, the driving impedance shown to the mixer, especially if the mixer is a passive one, can be too low, thus degrading the performance of the down-conversion.
US 2013/043946 describes a wireless device including multiple receivers to support different frequency bands, thus trying to reduce circuitry and cost. This solution includes a plurality of low noise amplifiers having outputs that are combined before the combined radio frequency signal is down-converted in a common mixer. Although this disclosure further reduces the consumption of circuit area and power, it does not disclose any possibility to compensate for different signal levels in the different frequency bands, and since received signal strength can vary considerably between the frequency bands, the suggested solution is not suitable for simultaneous multiple band reception. Further, the impedance level of the combined low noise amplifiers outputs will depend on the number of active low noise amplifiers, and as a consequence also the frequency response and the linearity of the circuit will vary in dependence of the number of received bands.